The present invention relates to a piezoelectric composite transducer or an array of piezoelectric composite transducers of an ultrasonic probe for use in sonars for detection of a target object in water, ultrasonic diagnostic apparatus for examination of a human body and others.
Conventionally, as a material of a piezoelectric transducer used in an ultrasonic probe of a sonar, ultrasonic diagnostic apparatus and others whose investigation subject is water and human bodies, PZT or lead titanate (Pb Ti O.sub.3) ceramics is being used because of having a high electro-mechanical coupling factor. However, since these piezoelectric transducer materials have an acoustic impedance of 25 to 35.times.10.sup.6 Kg/m.sup.2. s which is extremely higher than the acoustic impedance, i.e., about 1.5.times.10.sup.6 Kg/m.sup.2. s, of the water or human body, mismatch easily occurs to deteriorate the efficiency. Thus, recently, as the piezoelectric transducer material having a high electro-mechanical coupling factor and a low acoustic impedance close to that of the water or human body, a so-called piezoelectric composite transducer material is being studied which is composed of finely-divided piezoelectric ceramics and an organic material filling up spaces between the finely-divided ceramics. In addition, the piezoelectric ceramics is arranged so as to have a porous structure whereby the aforementioned characteristic can be obtained.
One example of the above-mentioned piezoelectric composite transducer are known by "W.A. Smith et al: Proc. IEEE 1985 Ultrasonics Symp. 642-647"or "Electronic Communication Academic Society Technical Research Report Shigaku Giho Vol. 83. No. 160 U.S. 83-30. The conventional piezoelectric composite transducer will be described hereinbelow with reference to FIG. 1.
In FIG. 1, a so-called 1-3 type piezoelectric composite transducer 53 is illustrated where a PZT piezoelectric ceramics 51 and an organic polymeric material 52 such as a silicon rubber and an epoxy resin are one-dimensionally and three-dimensionally coupled to each other. Generally, for manufacturing this piezoelectric composite transducer 53, one piece of piezoelectric ceramics is cut and finely divided by means of a dicing machine or the like so as to provide a reticulate configuration before filling up with the organic polymeric material 52, i.e., a silicon rubber, spaces between the cut piezoelctric ceramics 51. At this time, by adjusting the thickness of the edge for cutting the piezoelectric ceramics 51 and the cutting pitch, the volume ratio of the piezoelectric ceramics 51 is allowed to be controlled so as to attain a desirable characteristic. The electro-mechanical coupling factor of the piezoelectric composite transductor thus arranged can be obtained to be substantially equal to that of a single piezoelectric ceramics and further the acoustic impedance thereof becomes smaller than that of the single piezoelectric ceramics. piezoelectric ceramics 51 to 25% and the organic polymeric material, i.e., silicon rubber, is 75%, the acoustic impedance of the piezoelectric composite transducer 53 becomes about 8.8.times.10.sup.6 Kg/m.sup.2. s, thereby resulting in considerably improving the matching to the acoustic impedance of water or human body as compared with that of a single piezoelectric ceramics to heighten the transmission and reception efficiency of an ultrasonic wave.
The other conventional piezoelectric transducer using a porus piezoelectric ceramics is known by Japanese Patent Provisional Publication No. 63-78700. In this piezoelectric composite transducer, a PZT piezoelectric ceramics, for example, is arranged so as to have a porus structure to lower the acoustic impedance with decrease in the density. For example, a dispersion liquid is produced with a PZT ceramics powder and a PVA and a water soluble acrylic resin, acting as a binder, and then formed so as to have a sheet-like configuration, before burning with a vacancy rate of member to obtain a porous ceramics with a vacancy rate of 43%. A porous piezoelectric transducer can be obtained by performing the polarization process with respect to the porous ceramics. The acoustic impedance of this porous piezoelectric transducer results in being about 6.times.10.sup.6 Kg/m.sup.2. s and the electro-mechanical coupling factor thereof becomes about 63%, whereby the acoustic matching with respect to water or a human body becomes excellent as well as the above-mentioned piezoelectric composite transducer to improve the transmission and reception efficiency of an ultrasonic wave.
However, of the above-described conventional examples, the former is arranged as a composited member comprising the piezoelectric ceramics 51 and the organic polymeric material 52, and therefore, the limitation is imposed on approaching the acoustic impedance to that of water or a human body. That is, although the acoustic impedance is expressed as the product of the density and acoustic velocity, in practice the adjustment of the acoustic impedance more depends upon variation of the density rather than the acoustic velocity, and the density of the piezoelectric ceramics 51 is about 7 to 8 Kg/cm.sup.3 and the density of the organic polymeric material, i.e., silicon rubber, 52 is about 1 Kg/cm.sup.3. Thus, even if the volume ratio of the piezoelectric ceramics 51 is arranged to be small, the acoustic impedance is limited to about 7.times.10.sup.6 Kg/m.sup.2. s. In the case of further decreasing the volume ratio thereof, the volume ratio of the organic polymeric material 52 should increase, whereby the electro-mechanical coupling factor is lowered so as to deteriorate the entire characteristic thereof. Therefore, when compared with the case of the single piezoelectric ceramics, although the acoustic impedance can be closed to that of water or a human body, there is a problem that the acoustic matching is not yet satisfied. On the other hand, in the case of the latter, although the acoustic impedance can be lowered by increasing the vacancy rate of the porus piezoelectric ceramics, when the vacancy rate exceeds a given value, the mechanical strength becomes extremely weak to make easy damage thereof. Further, since the permittivity becomes extremely small, the electrical impedance becomes high to result in provide a problem that difficulty is encountered to meet the electrical matching. Thus, it is difficult to decrease the acoustic impedance to be smaller than a predetermined value and this cause the acoustic impedance to be difficult to be sufficiently close to the acoustic impedance of water or a human body, thereby resulting in impossibility to sufficiently satisfy the acoustic matching.